1. Field of the Invention
The invention relates to a security device for use on documents of value and the like.
2. Description of Related Art
A well-known security device is in the form of a surface relief hologram or other similar diffraction optical security device such as a Kinegram, Exelgram etc. which is formed as a hot stamp foil or the like on a carrier and can then be transferred by hot stamping onto the substrate to be secured, such as a banknote. Other embodiments of such security devices include tamper evident holographic labels and holographic threads for banknote applications. There is a continuing need to enhance the security of such devices and U.S. Pat. No. 5,310,222 describes such a device in which an embossed surface relief is de-metallised to form printed images located within a discontinuous metallisation behind the relief.
A further enhancement is described in EP-A-0680411 in which a luminescent material is included in an adhesive layer and/or a transparent layer.
One of the drawbacks with all these approaches is the effect of the underlying substrate onto which the security device is provided. This is particularly because non-security papers used in many security document applications (tickets, tax stamps, etc.) tend to be UV bright and naturally fluoresce strongly so masking the fluorescent feature, for example in EP-A-0680411, which is therefore only useful for UV dull banknote papers. High value security documents are usually based on UV dull papers or substrates ie they do not respond under UV light. These are expensive and for many security applications e.g. labels, substrates such as paper are used but these are optically active and fluoresce strongly under UV light. If a fluorescent material is included in one of the layers of the device, that material fluorescing in response to UV light, a problem arises when the underlying substrate is also sensitive to UV light and also fluoresces brightly. In this situation, the underlying fluorescence can obscure the desired fluorescence from the security device itself. We have attempted to increase the strength of the desired fluorescence by heavily loading the appropriate layer, typically the adhesive, but this is undesirable and can cause problems in coating and adhesion at the weight required.
In accordance with the present invention, a security device comprises an optically variable effect generating structure; an obscuring layer located, in use, between the optically variable effect generating structure and a substrate to which the device is secured; and a colour layer which exhibits one or more predetermined colors when viewed under certain viewing conditions from the side of the device remote from the obscuring layer and wherein the obscuring layer is substantially opaque at least under the said certain viewing conditions.
We have overcome the problems set out above by including an obscuring layer which is substantially opaque under the said certain viewing conditions. By this we mean that the layer is sufficiently opaque to obscure a conflicting response due to the underlying substrate under the said viewing conditions (for example UV irradiation).
The obscuring layer may also include one or more colors different from the predetermined colour(s) of the colour layer. The effect of this is that when the security device is viewed under the said certain viewing conditions, the predetermined colour(s) of the colour layer will be seen whereas under other viewing conditions the one or more colors of the obscuring layer will be seen. In both cases, these colors will be seen through the optically variable effect. The result is a highly secure device which exhibits different colors when viewed under different viewing conditions and is very hard to counterfeit.
The optically variable effect generating structure can take any conventional form. Typically, the optically variable effect generating structure comprises first and second layers defining a surface relief at their interface. The second layer may comprise one or more layers of relatively high refractive index material such as zinc sulphide, titanium dioxide, and vanadium pentoxide to provide a partially reflective/partially transmissive layer. Alternatively, a discontinuous, fully reflective material such as a partial demetalisation may be provided at the interface, as obtained, for example, from an aluminium layer. This partial demetallised metal (e.g. aluminium) layer can be created using a number of demetallisation processes known in the art. Typically, the first layer will comprise a lacquer with an embossed surface relief while the reflective layer comprises a partially metallised surface or refractory material whilst the additional colour and obscuring layers will comprise of varnish and/or adhesives.
In most cases, we envisage a simple structure in which the optically variable effect generating structure, colour layer and obscuring layer are provided one above the other but it is possible for one or more intermediate layers to be provided.
In the preferred examples, the xe2x80x9ccertain viewing conditionsxe2x80x9d correspond to irradiation under UV light with the resultant colour(s) being in the form of fluorescence visible either within the visible wavelength range (for ease of authentication) or invisible to the human eye but detectable outside that range (for example in the infra red for machine reading). However, the material contained by the second layer could instead be responsive to radiation at other wavelengths, for example infrared phosphorescence from visible or invisible markings.
In order to secure the device to a substrate, it is necessary to include an adhesive. This could be provided during the securing step, either by placing the adhesive on the substrate or on the obscuring layer (or a layer joined directly or indirectly to the obscuring layer) but conveniently an adhesive layer is provided as part of the security device. In the most preferred approach, the third layer itself constitutes an adhesive.
Preferably, the obscuring layer is substantially continuous. This has the advantage that the entire underlying surface of the substrate onto which the security device is provided is obscured, at least under said certain viewing conditions. However, we also envisage a discontinuous obscuring layer. In this situation, some effects of the underlying substrate would remain visible through the discontinuous reflective material while those areas of the substrate covered by the obscuring layer would not.
The obscuring layer may comprise a plurality of sections, preferably in the form of stripes which may be abutted beside each other or even overlap to generate a continuous layer. Those sections may have the same or different colors by including one or more pigments in the sections.
The optically variable effect will typically be a hologram but other diffractive effects could also be used such as pure diffraction grating structures, Exelgrams and Kinegrams as known in the art. The optically variable effect itself can generate one or more patterns which may have graphic or pictorial forms or comprise symbols and these may be in the form of a single such representation or multiple representations. Furthermore, the patterns may be visible to the naked eye or only after magnification.
The colour layer may exhibit a simple colour or colors which are visible through the discontinuous reflective material or transparent refractory layer and hence the optically variable effect. In more sophisticated approaches, the colour layer may exhibit one or more patterns when viewed under the appropriate viewing conditions. These patterns could be linked with the substrate on which the security device is provided (for example a symbol or the like which also appears on the substrate) or in the preferred approach relate to the optically variable effect. For example, the patterns generated by the colour layer and the optical variable effect could be the same.
It will be appreciated that typically the optical variable effect will be readily detectable to the naked eye providing a further level of security while the presence of the colour(s) and/or pattern(s) of the colour layer will only be detectable under the certain viewing conditions such as UV irradiation. This therefore provides a second, covert level of authentication. Where the colour layer generates a plurality of substantially identical patterns, these could be visualised with a micro-lens array having similar dimensions as described in more detail in EP-A-0698256 using for example fluorescence to increase feature visibility to the human eye, so aiding verification.
The material incorporated in the colour layer can be chosen from a wide variety of materials which exhibit one or more predetermined colors under the certain viewing conditions. The use of UV or IR radiation has already been mentioned above to stimulate fluorescence or phosphorescence. Other materials sensitive to different energy input forms include thermochromic inks responding to temperature change, photochromic inks responding to selected light wavelength input, optically variable inks, magnetic inks responding to electromagnetic fields, phosphor inks, anti-stokes materials and the like.
It would also be possible to include one or more of these inks in the obscuring layer either alone (if sufficient opaqueness is achieved) or in combination with a further pigment or the like which achieves the substantially opaque nature of the obscuring layer.
The layers can be provided in any conventional method using coating or printing techniques well known to persons of ordinary skill in the art. Examples of typical printing techniques include lithographic, letterpress, intaglio/gravure, flexography, ink jet, dye diffusion and toner deposition.
The security device can be used on a wide variety of substrates but is particularly suitable for documents of value of many different types. Examples include vehicle licences, tax discs, certificates of insurance, and the like. More generally, the invention is applicable to visas, passports, licences, cheques, identity cards, plastic cards, banknotes, tickets, bonds, share certificates, vouchers, passes, permits, electrostatic photocopies, electrostatic laser printed materials, brand authentication labels and other documents and packaging, serial numbering slips, quality control certificates, bills of lading and other shipping documentation, legal documents and tamper evident labels.